Selectivity adjustment coupling network



R. w. TURNER May 21, 1946.

' S'ELECTIVITY ADJUSTMENT QOUPLINGVNETWORK Filed May 9, 1942 2 Sheets-Sheet 1 J r"-----w RP v 9 mm m u IW A P May 21, 1946. R. w. TURNER SELECTIVITY ADJUSTMENT COUPLING NETWORK Filed May '9, 1942 2 Sheets-Sheet z ATTORNEY m 21,. ms

2,400,857 SELECTIVITY ADJUSTMENT COUPLING NETWORK Robert w. Ma. East Orange, N.

Federal Telephone and Radio Co corporation of Delaware J., assignmto rporaiion, a"

Application May a, Masai-1n No. 442,336

50lalms. This invention relates to wave coupling networks and more especially to such networks as are used for tuned couplings and are of adjustable selectivity. 7

A principal object of the invention relates to ea wave coupling arrangement for circuits which are adjustable as to. selectivity while being maintained tuned to a given freouency. Another principal object is to provide a tune interstage coupling arrangement for high frequency amplifiers whereby a constant capacity reactance is maintained in while permitting adjustment as to selectivity.

Another object is to provide an improved coupling arrangement for the intermediate frequency amplifier section of a superheterodyne receiver or the like which coupling arrangement is adjustable as to selectivity while maintaining the resonant frequency constant.

A feature of the invention relates to a plural stage wave amplifier wherein the interstage coupling is of adiustableselectivity while being maintained at a desired resonant frequency. in conjunction with specially designed switch means to compensate for, change. of gain tending to result from changes in selectivity.

Another feature relates to a tuned coupling arrangement particularly well suited to couple the stages of an intermediate frequency amplifier, wherein the tuned unit is divided into a plurality of tuned sections which are capacitively coupled, the coupling capacity being so arranged that the degree of coupling can be varied to vary the selectivity. while the tuning is not materially affected.

.A further feature relates to a tuned coupling arrangement for intermediate frequency ampliflers wherein the tuning capacities of two resonant circuits consists of two sections in series, one of which sections is common to the two resonant circuits, and which is in the form of a condenser bank and provided with a switching means whereby the shunt-series combination of the condensers can be varied.

A.still further features relates to the novel organization and interconnection of parts which constitute animproved intermediate frequency coupling arrangement for superheterodyne receivers and the like.

7 Other features and advantages not particularly set forth will be apparent after a consideration of the following descriptions and the appended claims.

While the invention will be described as em bodied in an intermediate frequency amplifier ofa superheterodyne receiver, it will be underthe resonant circuits stood that such is done merely forexplanatory purposes and .not by way. of limitation to such 1 uses or to the specific apparatus and parts re- 1 ferred to. Accordingly in the drawings, Fig. 1 is a schematic circuit diagram of a portion of a wave amplifying system embodying the invention.

Figs. 2, 3 and 4 vare respectively simplified schematic diagrams of the coupling and tuning sections of Fig. 1 in three different operational settings. i I

Referring to Fig. 1, there is shown inschematic detail the intermediate frequency amplifier portion of a superheterodyne receiver wherein the incoming intermediate frequency signal from the line L, which is connected to the output of a II, which are shunted by the usual .by-pas s condenser ll. v

The resistor 10 is to complete the direct current path from grid to ground and has no eii'ect on the performance of the tuned circuits or coupling circuits. Also, a by-pass condenser I5 is connected from grid 5 and the low potential end of the inductance 2| to ground.

As a result of the connections above described. the incoming intermediate frequency signal is amplified by tube 3,- which is'coupled to succeeding I. F. amplifier and detector and audio amplifler stages represented schematically by the block 4 It. The output load circuit ll.

' In such systems, it is highly desirable to be of the coupling beand the succeeding.

. able to vary the selectivity tween the amplifier tube I I stages it. However, when varying the degree of selectivity it is desirable that the intermediate frequency tuning should be maintained constant. and at the same time it is desirable to maintain a uniform overall gain in the amplifier system. Accordingly, I have devised a novel form of coupling and switching arrangement whereby these desirable results are obtained. For this purpose, the couplingunit between the tube 3, and the stage II comprises a network divided into a plu- 55 raiity of units it, It, 20, each unit including a of i8 is applied to any form of a line shown in Fig. 1.

tuning inductance coil'2l, 22, 28, with associated tuning condenser sections; one section for each coil comprising a fixed condenser 24, 25, 28, and its parallel trimmer condenser 21, 28, 29; the other section for the coils consists of two banks of fixed condensers 88, 8|, 82, and 84, 85, 88.

The condenser bank consisting of condensers 80,,8l and..82 is adapted to be connected between the condensers 24, 21 and 25, 28, in various combinations to provide different coupling coefficients between the units l8 and I9. This variable con- The gain control switch section l8 has a set of brushes 88-42 which cooperate with a rotatable metal commutator 48 adapted to assume three different positions. In the first position as shown, cathode 9 is connected to ground through resistor I, brush 89, segment 48, brush 4|, resistor l8. In the second position of the switch, cathode 9 is connected to ground through resistor brush 39, commutator 48, thence in parallel through brushes 40 and 4| and resistors l2, 18. In the third position, cathode 9 is connected through resistor brush 89, segment 48, brush 88 to ground. Thus in the three successive positions of, switch section II), the. negative bias of grid 2 is reduced whereby the gain of tube 8 is correspondingly increased.

The condenser bank 89, 8|, 82, is connected to switch section 88 so that the coupling coemcient between condensers 24, 21 and 25, 28 is decreased in three successive steps; and likewise switch section 81 connects the 'condenser bank 84, 85, 88, in various combinations, so that the coupling coeflicient between condensers 25, 28 and-28, 29 is likewise decreased in three steps. This decrease in coupling tends to result in a proportionate reduction in gain in the output of the amplifier.

- However, since the gain of tube 8 is increased in proportion to the reduction in gain in the coupling network, the overall gain of the amplifier remains uniform while allowing the changes in selectivity. In order to accomplish this, it is necessary to proportion the condensers of each bank properly with respect to each other and with respect to the associated condensers 24--21, 25-28,

- 28-29, so that the overall capacity reactance in 21 and 28, respectively, for the purpose of maintaining constant tuning. -A similar arrangement applies in the case of the circuits l9 and 28 and th second bank of condensers except that condenser 85 is short-circuited.

In the third position of the'gang switch (Fig. 4), the condensers are connected in the same general relation as in Fig. 8 with the exception that theooupling condensers 82 and 88 are of higher capacity than the corresponding coupling condensers in Fig. 8 and hence the coupling coemcients are reduced. I

In order that the switching function of the two switch sections 88 and 81 may be clearly underthe coupling network remains uniform while reducing the coupling coeflicient. The manner in which the various condensers of the coupling network are connected into circuit-in the three positime of the garg'switch are represented schematically in Figs. 2, 8, and 4. Referring to Fig. 2, it will be noted that the coupling coeflicient between the circuits l8 and I9 is controlled by the series condenser combination 82 and 8|. Simi-- larly the series condenser combination 88 and 85" .controls the coupling coeflicient between circuits l9 and 28. The tuning of each'circuit is at the desired intermediate frequency. In position 2 of of the first bank is connected between ground the switch as shown in Fig.8, the condenser via brush 45, commutator segment 85, brush 88.

The common direct connection between conductors 88 and 89 is returned to ground via brush 41, segment 58, brush 49, condenser 82, strap 58. condenser 8|, brush 5|, segment 52 and brush 58. Likewise, conductor 58 is directly connected to conductor 55 via brush 58, segment 51 and brush 58. This direct connectionbetween conductors 54 and 55 is returned to ground via brush 59, segment 58, brush 6|, condenser 88, strap 82, condenser 85, brush 88, segment 84 and brush 85. In second position, conductor 88 is connected to conductor 89 throughtwo condensers 8|, 82, in series as follows. Conductor 88, brush 41, segment 55, brush 49, condenser 82, strap 50, condenser 8!. brush 5|, segment 45 and brush- 48. The common connection between condensers 8| and 82 is connected to ground through condenser 88 via brush 88, segment 52, brush 58. Likewise, in second position of section 81, conductor 54 is connected to conductor 55 but only through condensers 88 since condenser 85 is short-circuited at 88and 51 and the common connection of these two condensers is returned to ground through condenser 84. In the third position of the switch,

Conductor 88, brush 41, segment 48, brush 81,

condenser 80, condenser 8|, brush. 5| segment 45, brush 48, conductor 89. The common connection between condensers 88 and 8| is returned to ground through condenser 82 via brush 49, segment 52, brush 58, brush 88, segment 52. Merely for illustration, condensers 88, 8| and 84, 85, may each be of .01 mfd. while condensers 82 and 88 can be .02 mfd. It will be understood of course that any other values of the condensers maybe employed so that in conjunction with the remaining electrical constants of the coupling network, the capacity reactance remains substantially constant while varying the effective capacity coupling between the various sections of the network. It will be understood of course that while the invention shows an amplifying system employing a single intermediate frequency amplifying stage, between the amplifier 8 and the first to sections 88 and 81 in which event these additional sections may be operated from the same gang shaft as the sections ll. 88 and 81.

Various changes and modifications may be made in the disclosed embodiment without departing from the spirit and scope of the invention.

What I claim is:

1. A tuned coupling arrangementior electric wave amplifiers comprising a pair of tuning inductances, each having a high potential end and a low potential end, a pair of tuning condensers, one end of each is connected to said high potential ends, a bank of fixed condensers for sup-- plementing said pair of condensers in tuning said inductances, and plural-position switch means eflective in one setting to connect said pair of condensers directly in series between the said high potential ends while connectin a plurality of condensers of said bank from the common junction of'said pair of tuning condensers to said low potential ends; said switch means .belng effective in another setting to connect one condenser of said bank in series between said pair of tuning condensers.

2. A tuned coupling arrangement for electric wave amplifiers comprising a pair of circuits to be coupled, each including an inductance and a tuning condenser, a bank of fixed condensers, and a multi-position switch eflective in one setting to connect the condensers of. said bank in simple series-shunt across the high and low potential terminals of the coupling circuit and effective in second and third settings to connect the condensers in said bank in a T or shuntseries relation across said ends, the second and third settings producing difierent degrees of selectivity in the coupling as compared with the first setting but without changing the tuning.

3. An electric wave amplifier comprising a tuned network adapted to couple two vacuum tubes in series, including two tuned sections and a variable capacitance unit coupling the sections, said unit comprising a plurality of fixed capacities and stage switch means for connecting said capacities in coupling relation to the sections to form a series of coupling capacity values, thereby varying the couplingcoefllcient between said sections, and the gain of the input tube; gain compensating mean 'i'or simultaneously varying the gain of. the input tube in the opposite sense, including a plurality of resistances and stage switch means for connecting said resistances in circuit with an electrode of the input tube to form a plurality of varying resistance values in said circuit, arranged to produce changes in the tube gain compensating for gain changes produced by similar stage variations in the capacitance unit; and means iOr synchronously operating both of said stage switch means arranged to maintain uniform input tube gain. a

4. An amplifier as set forth in claim 3, in which the capacitancestage switch means i arranged to connect said fixed condensers in series tuning position between said sections.

5. A tuned coupling arrangement for electric wave amplifiers comprising a pair otcircuits to be coupled, a condenser bank including two condensers or different capacity value, and selective switch means for connecting said bank in coupling and tuning relation to said circuits, including means for connecting either of said two condensers in series tuning relation and the other in coupling relation between said circuits.

ROBERT w. TURNER. 

